The present invention relates to a curable composition of polychloroprene, chlorinated polyethylene, or epichlorohydrin rubber, using a novel organic accelerator.
2-Mercaptoimidazoline has been widely used as an indispensable accelerator for curing polychloroprene, since it has an excellent accelerating effect and gives a tightly cured polychloroprene vulcanizate having excellent mechanical properties suitable for practical use.
The accelerating effect of 2-mercaptomidazoline is so superior that it is very important to the polychloroprene rubber industry. This is evident from the huge amount (4,000 tons or more per year in 1970) of 2-mercaptoimidazoline consumed all over the world.
However, it has been recently pointed out that 2-mercaptoimidazoline generates gaseous hydrogen cyanide in the course of the heat-treatment or the thermal curing process of polychloroprene composition.
This toxicity of 2-mercaptoimidazoline such as the formation of hydrogen cyanide brought about a trend to stop the manufacturing of this chemical by some major manufacturers, and the usage thereof was strictly restricted in several countries. A tendency to avoid 2-mercaptoimidazoline has become remarkable in the last few years.
This movement has caused a crisis in the polychloroprene industry because of the shortage of supply of this chemical, and the development of a new accelerator which is able to take the place of 2-mercaptoimidazoline has become an urgent problem.
To those skilled in the art, the development of newer accelerator for halogenated elastomers is extremely difficult. This is the reason why the curing mechanism of the elastomers is still entirely unknown and, furthermore, so complicated that the curing mechanism is peculiar to each halogenated elastomer. Since each individual curing system is so complicated and each halogenated elastomer behaves differently, most curing systems for chlorinated elastomer have not been applicable to fluorinated or brominated elastomers, and vice versa.
This failure of one curing system not being adaptable for use with different halogenated elastomers also extends to some compositions using the same halogenated elastomer but of a different grade. For example, triallylisocyanurate which is used as an accelerator for chlorinated polyethylene containing 30% by weight of Cl does not act as a useful accelerator for another chlorinated polyethylene containing less than 10% by weight of Cl. Such circumstances imply that it would be impossible to develop a novel curing chemical from the prior art.
This is the very reason why a novel curing chemical for halogenated elastomers has never been developed through analogical approach.
Investigation of the present inventors have overcome the above situation to develop a novel accelerating system which surprisingly displays an excellent accelerating effect for curing of polychloroprene and chlorinated polyethylene as well as an epichlorohydrin elastomer which is comparable with or superior to 2-mercaptoimidazoline and, as a very important advantage, which does not form any toxic gases.